The novel is Shattered Earth, an alternate history science fiction novel.

Summary:
Advanced humans, called Terrans, leave Earth when its threatened by a massive meteor. The remaining humans on Earth fall back into primitiveness. The advanced humans, and a group of aliens, the Ragnor, involve Earth in an interstellar war. Thousands of years later, the humans of Earth decide to do something about this. Interstellar politics will never be the same again once Earth is done with their revenge.

These Terrans involve themselves throughout human history. Some play as King Arthur and Merlin. Another is a scientist, whose name is Galileo, and he gets into all sorts of trouble for challenging official orthodoxy about the sun, the Earth, and which one is revolving around the other. Then, we have Amelia Earhart, who has a very valid reason for disappearing. When an alien spaceship gets shot down over Roswell in 1947, President Truman orders the creation of Area 51, Project Blue Book and Sign, and has the United States embark on a plan that will culminate 60 years in the future. Nothing will ever be the same again when the United States gets involved in interstellar politics. But, the whole plan backfires when Earth ends up the worse for wear over it. Other species find out what Earth did, since no one else was bold enough to even try, so they want to help bring down the Ragnor once and for all. The new President of the United States imagines a galactic federation or a republic, but, none of the other species wants anything to do with it. Once they destroy the Ragnor’s technology, they leave the Ragnor to ponder why all the other species hated them enough to attack them. Will the galaxy remain at peace?

Sounds very Stargate SG1 and Stargate Atlantis - like. In that series the Ancients evolved on Earth, left earth, seeded at least 5 galaxies with stargates, fought wars and then ascended into a higher plane of existence. Remaining ancients remained on Earth and became important figures through history and legend, whilst the 2nd evolution of humanity took place.

I wasn't thinking about SG-1 when I wrote it, but, I can see how one might think it's similar. But, my advanced humans never go to a higher plane of existence, they just interfere a lot in human history. The aliens, the Ragnor, who interfere on Earth too, are the ones who cause the problems at Salem and Roanoke, and give Isaac Newton all of his ideas. They aren't the little brown aliens in this novel, even though those particular aliens do show up near the end. Some Terrans are our more famous science fiction authors, science fiction TV producers, and even a current talk show host who is a wanted felon in other parts of the galaxy(her name is changed so I won't get sued! lol)